The neurotransmitter signals to the brain that the body has made a nutritional change. The brain sends signals to the pancreas, which will release insulin.
Exercising on an empty stomach is what’s known as fasted cardio. The theory is that your body feeds on stored fat and carbohydrates for energy instead of food you’ve recently eaten, leading to higher levels of fat loss. If the idea of taking a day or two off each week is hard for you to manage, consider treating these days as active rest. If you want to lose weight, consider two days of moderate activity and two days of vigorous aerobic activity or high-intensity-interval-training .